TCP three-way handshake: in-depth understanding and C# example implementation

In computer network communications, TCP (Transmission Control Protocol) plays a vital role as a connection-oriented, reliable, byte-stream-based transport layer communication protocol. TCP uses a three-way handshake mechanism to ensure that a stable and reliable connection can be established between two communication nodes. This article will explore the principle of TCP three-way handshake and show how to implement this process in practical applications through C# sample code.

1. TCP three-way handshake principle

The TCP three-way handshake is the standard process for establishing a TCP connection in the TCP/IP protocol. It ensures that both parties in data communication can synchronize sequence numbers, laying the foundation for subsequent reliable data transmission. The three-way handshake process can be summarized into the following three steps:

1. First handshake: The client sends a SYN (synchronization sequence number) segment to the server. This segment does not contain application layer data, but only a SYN flag bit, which is used to synchronize the sequence number. At this point, the client enters the SYN_SENT state and waits for confirmation from the server.
2. Second handshake: After receiving the SYN segment from the client, the server will respond with its own SYN segment, which also does not contain application layer data, but contains two flags: SYN and ACK (confirmation sequence number is valid). The ACK flag is used to confirm that the SYN segment from the client has been received, while the SYN flag indicates that the server wants to establish a connection. At this point, the server enters the SYN_RCVD state, waiting for the client's confirmation.
3. The third handshake: After the client receives the SYN+ACK segment from the server, it will send an ACK segment to the server to confirm that it has received the SYN segment from the server. At this point, both the client and the server have entered the ESTABLISHED state, indicating that the TCP connection has been successfully established and both parties can start transmitting data.

2. Purpose of TCP three-way handshake

The main purposes of the TCP three-way handshake include:

1. Synchronize the initial sequence numbers of both parties: Through the three-way handshake, the communicating parties can synchronize their respective initial sequence numbers, providing a reliable sequence number basis for subsequent data transmission.
2. Exchange TCP window size information: During the connection establishment process, the two parties will exchange TCP window size information to enable effective flow control during data transmission.
3. Confirm the receiving and sending capabilities of both parties: Through the three-way handshake, both parties can confirm that the other party has the ability to receive and send data, ensuring the reliability of subsequent data transmission.

3. C# sample code to implement TCP three-way handshake

Although the TCP three-way handshake is automatically completed in the underlying network protocol stack, we can simulate this process through C# code to deepen our understanding of the principle of TCP three-way handshake. The following is a simplified C# example that shows how to use the Socket class to simulate the connection establishment process between the TCP client and server.

Server-side code

 using System; using System.Net; using System.Net.Sockets; using System.Text; public class TcpServer { public static void StartServer(int port) { IPAddress ipAddr = IPAddress.Any; TcpListener server = new TcpListener(ipAddr, port); server.Start(); Console.WriteLine("Server started..."); while (true) { Console.WriteLine("Waiting for a connection..."); TcpClient client = server.AcceptTcpClient(); Console.WriteLine("Connected!"); // 在实际应用中，这里会开启一个新的线程来处理客户端连接// 为了简化示例，我们直接在这里处理连接NetworkStream stream = client.GetStream(); // 假设这是第二次握手，服务器发送SYN+ACK // 但实际上，这一步是由操作系统自动完成的// 接收客户端的第三次握手ACK byte[] buffer = new byte[1024]; int bytesRead = stream.Read(buffer, 0, buffer.Length); if (bytesRead > 0) { Console.WriteLine($"Received {bytesRead} bytes from client."); // 发送响应给客户端，表示连接已建立string response = "Connection established"; byte[] msg = Encoding.ASCII.GetBytes(response); stream.Write(msg, 0, msg.Length); } client.Close(); } } static void Main(string[] args) { StartServer(12345); } }

Note: The server code above does not actually simulate the TCP three-way handshake process directly, because the TCP three-way handshake is automatically completed by the underlying network protocol stack of the operating system. The example here is mainly to show how to use the C# Socket class to establish a TCP connection and send and receive data after the connection is established.

Client code

 using System; using System.Net.Sockets; using System.Text; public class TcpClientProgram { public static void StartClient() { TcpClient client = new TcpClient("127.0.0.1", 12345); NetworkStream stream = client.GetStream(); // 第一次握手由操作系统自动完成，这里我们直接发送数据作为模拟的第三次握手ACK string message = "Hello, Server!"; byte[] data = Encoding.ASCII.GetBytes(message); stream.Write(data, 0, data.Length); Console.WriteLine("Sent: {0}", message); // 接收服务器响应data = new byte[256]; string responseData = String.Empty; int bytes = stream.Read(data, 0, data.Length); responseData = Encoding.ASCII.GetString(data, 0, bytes); Console.WriteLine("Received: {0}", responseData); stream.Close(); client.Close(); } static void Main(string[] args) { StartClient(); } }

In this client example, we also did not directly simulate the TCP three-way handshake process. However, by sending data to the server and receiving a response, we simulated the data transmission process after the TCP connection is established.

4. In-depth understanding of TCP three-way handshake

Although the above C# example does not directly show the specific implementation of the TCP three-way handshake, it helps us understand the basic process of TCP connection establishment and data transmission. In actual applications, the TCP three-way handshake is automatically completed by the underlying network protocol stack of the operating system without the need for manual intervention by the programmer. However, understanding the principle of the TCP three-way handshake is crucial for developing high-performance and high-reliability network applications.

The TCP three-way handshake ensures that both parties in data communication can synchronize sequence numbers and confirm each other's receiving and sending capabilities. This mechanism is one of the cornerstones of the TCP protocol reliability and provides a solid foundation for subsequent data transmission.

V. Conclusion

This article explores the principle of TCP three-way handshake in depth, and shows how to simulate the process of TCP connection establishment and data transmission at the application layer with C# sample code. Although the sample code does not directly implement the specific steps of TCP three-way handshake, it helps us understand the basic process of TCP connection establishment and data transmission, as well as how to use the Socket class in C# for network programming. I hope this article can help readers to have a deeper understanding of the principle of TCP three-way handshake and its implementation in practical applications.